Add FastAPI backend and React frontend subprojects

Backend:
- FastAPI app with 17 REST endpoints covering dashboard, monitoring,
  media index, file browser, and jobs
- Reuses existing clients/domain/services unchanged
- pydantic-settings config, dependency injection, CORS setup
- Auto-generated OpenAPI docs at /docs

Frontend:
- Vite + React + TypeScript SPA
- @tanstack/react-query for data fetching with polling
- ag-grid-react for media table and file browser
- recharts for monitoring charts
- Tailwind CSS styling
- 4 pages: Dashboard, Monitoring, Media, File Browser
- Typed API client matching all backend endpoints

Also:
- docs/MIGRATION_PLAN.md with full architecture plan
- Updated .gitignore for both subprojects
- Streamlit app preserved for now (can coexist)

backend/pyproject.toml

@@ -0,0 +1,24 @@
+[project]
+name = "media-library-viewer-backend"
+version = "0.1.0"
+description = "FastAPI backend for Media Library Viewer"
+requires-python = ">=3.11"
+dependencies = [
+  "fastapi>=0.100",
+  "uvicorn[standard]>=0.20",
+  "pydantic-settings>=2.0",
+  "paramiko>=3.0",
+  "requests>=2.28",
+  "python-dotenv>=1.0",
+  "pandas>=2.0",
+]
+
+[project.optional-dependencies]
+dev = ["httpx", "pytest", "pytest-asyncio", "ruff"]
+
+[tool.ruff]
+line-length = 120
+target-version = "py311"
+
+[tool.ruff.lint]
+select = ["E", "F", "I", "W"]